This invention relates to electric power cable insulated with a polyethylene composition having an improved resistance to water trees. A typical electric power cable generally comprises one or more conductors in a cable core that is surrounded by several layers of polymeric material including a first semiconducting shield layer, an insulating layer, a second semiconducting shield layer, a metallic tape or wire shield, and a jacket.
These insulated cables are known to suffer from shortened life when installed in an environment where the insulation is exposed to water, e.g., underground or locations of high humidity. The shortened life has been attributed to the formation of water trees which occur when an organic polymeric material is subjected to an electrical field over a long period of time in the presence of water in liquid or vapor form. The formation of water trees is believed to be caused by a complex interaction of the AC electrical field, moisture, time, and the presence of ions. The net result is a reduction in the dielectric strength of the insulation.
Many solutions have been proposed for increasing the resistance of organic insulating materials to degradation by water treeing. One solution involves the addition of polyethylene glycol, as a water tree growth inhibitor, to a low density polyethylene such as described in U.S. Pat. No. 4,305,849. An improvement in electrical performance was described in U.S. Pat. No. 4,440,671. The combined teachings of these patents, however, left room for improvements in processability, such as scorch resistance and sweat-out.
Various additives have been used in combination with polyethylene to achieve desired characteristics in the material. As discussed in greater detail below, it has been difficult to achieve the proper balance of additives due to undesirable interactions and competing effects. Phenolic antioxidants, such as the thiobis compounds discussed herein, have been added to polyethylene formulations to improve scorch resistance. For example, the phenolic antioxidant, 4,4′-thiobis(2-t-butyl-5-methylphenol) (TBM6), has been used as an additive in polyethylene compositions. TBM6, however, has generally been used at levels of 0.2% or lower. At higher levels, there is a serious issue with long-term storage of the material due to the TBM6 blooming or sweating out to the surface of the material.
Blooming, or sweating out, is the tendency of a component of a polymer formulation to migrate to the surface of the product. Sweating out is a process that occurs over time, usually becoming a problem after several weeks, and the problem is increased when the product is exposed to temperatures above or below the ambient temperature. Sweating out is undesirable for a number of reasons. When a component sweats out, there is a reduced amount of the component remaining within the product, and the remaining component has a reduced ability to perform its desired function within the product. Additionally, the presence of component on the surface of the product can cause an undesired interaction with adjacent products or materials or can lead to a loss of other desired properties of the product.
The conventional method for avoiding sweating out by an antioxidant component is to add the component to the formulation only in an amount small enough so that the component will not sweat out when the formulation is exposed to elevated temperatures. This will limit the properties of the resin which the antioxidant is supposed to retain and/or enhance. Alternatively, the problem of sweating out may be dealt with by limiting the storage life of the material or by accepting the material with deficiencies.